1. Field of the Invention
The present invention relates to a radio receiving apparatus for use in, for example, a remote keyless entry (abbreviated to RKE in some cases) system which is intended to successfully receive, at least once, a single radio signal which is transmitted several times at predetermined intervals.
2. Description of the Related Art
A typical radio receiving apparatus, such as those adopting the diversity system, uses two or more antennas for successful reception of a signal. A radio receiving apparatus which uses two or more antennas carries out diversity switching for selecting a signal received with the best conditions from among those signals received by the respective antennas. This diversity switching, however, may require two or more detector circuits and a select switch, leading to a complicated and large circuit of the radio receiving apparatus, which thus becomes expensive. To address this problem, Japanese Unexamined Patent Applications Publication Nos. 2002-26785 and 2002-152097 disclose a system where diversity switching is not carried out despite the fact that two or more antennas are used.
In the system disclosed in Publication No. 2002-26785, two CDMA RF signals received by the respective antennas are converted to respective IF signals. One of the IF signals is delayed and then combined with the other IF signal. These combined signals are demodulated thereafter.
In the system disclosed in Publication No. 2002-152097, two CDMA RF signals received by the respective antennas are converted to respective IF signals, which are both delayed through a SAW filter and then combined with each other. These combined signals are demodulated thereafter.
Advantageously, neither of the systems disclosed in Publication Nos. 2002-26785 and 2002-152097 perform diversity switching, contributing to a less expensive radio receiving apparatus with fewer components.
No literature is available which introduces a radio receiving apparatus that neither uses the CDMA system nor carries out diversity switching. Furthermore, there is no known literature which discloses an RKE receiver using the diversity system.
The systems disclosed in Publication Nos. 2002-26785 and 2002-152097 are based on a feature of the CDMA system, namely, the capability of decoding (decrypting) signals which time-overlap or frequency-overlap each other. For this reason, it is difficult to apply the disclosed systems to radio receiving apparatuses which use FSK modulation and ASK modulation, such as RKE receivers.
Furthermore, diversity switching, which readily causes the receiving apparatus adopting it to become large for the reasons described above, is not suitable for a radio receiving apparatus, such as an RKE receiver, whose function is so simple as to handle only a small amount of communication information and which therefore must be inexpensive.